AEOLIAN BEDFORMS ON MARS: A SUMMARY OF VIEWS FROM THE SURFACE

Aeolian bedforms have been recognized in views returned from the martian surface by the Viking, Pathfinder, and Mars Exploration Rover missions. Analyses of these features commonly has focused on: (1) classification and origin (as dunes, ripples, drifts, or combinations); (2) activity/inactivity in the current surface environment; (3) particle size-frequency (speculative where image resolution is limited) and implications for transport distance, source, and history; (4) current or past formative wind direction(s) from orientations; and (5) constraints on wind shear stress from comparing particle size-frequency with threshold-of-motion wind tunnel experiments. Transverse bedforms are seen at all sites. On the basis of size and lack of slip faces, most are ripples. True dunes, while common on Mars, have been avoided by landed missions for safety reasons, are thus rare within landing ellipses, and have not been studied well from the surface. Ripples have been trenched by the MER rovers, revealing some details of how particle size-frequencies differ between bedform surfaces and their interiors. Microscopic Imager (MI) views of ripple surfaces and trenched interiors reveal some similarities between the larger martian ripples and terrestrial ripples where sand supply is poorly sorted or bimodal. From previous work such ripples form with the coarsest grains concentrated in a surface layer and scattered through the upper parts of the ripple interior. At Meridiani, smaller ripples of almost pure basaltic 50-150 micron sand are found within aeolian traps (e.g., floors of small craters and troughs), in some cases aligning with bright wind streaks known to be transient features; these ripples likely are currently active. Along the Opportunity traverse south of Endurance crater, larger ripples display banding on east faces, suggesting induration and erosion. The presence of potentially active and inactive windblown particles and their respective bedforms within the same area (reinforced by observations of wind-related changes to older rover tracks) highlights the importance of competition between the frequency of strong, mobilizing winds and the rate of surface induration processes.